The present invention relates to an optical information-recording medium capable of optical recording/reproduction of information and more specifically relates to an optical information-recording medium capable of optical recording/reproduction of electronic information and satisfactorily recording visible information on the label surface with use of a laser beam.
The significance of electronic information-recording media used for recording, storage and rewriting of a lot of information as computer peripherals is ever increasing in recent years. Of various types of such media, optical information recording media with which recording and reproduction are performed with use of a laser beam are attracting attention as large volume recording media due to their capability of high-density information recording, storage and reproduction. Examples of such optical information-recording media include those of phase-changing type represented by CD-RW and DVD-RW, and those based on organic dyes represented by CD-R and DVD-R.
The content of the electronic information recorded in such an optical information-recording medium can be confirmed via optical reproduction. On the other hand, it is also important, from the viewpoint of information management, to display the content of the electronic information on the surface of the medium as visible information. As such display methods, there have been conventionally adopted, for example, the method of printing and recording the electronic information content on the label surface of the medium (the surface opposite to the surface for recording electronic of information) with use of an inkjet printer or thermal printer, the method of attaching a label printed with the electronic information content. Of these and others, a method of recording visible information in the label surface of the medium with use of a laser beam (Patent Document 1), and a method of further attaching a sheet, on which letters are printed, on the recording layer for visible information (Patent Document 2) have been reported recently. Such methods are considered promising because they can record visible information without exerting any mechanical influence on the medium.
FIG. 2 is a diagram to describe the configuration of a conventional optical information-recording medium in which visible information is recorded on the label surface side with use of a conventional laser beam as described, for example, in Patent Documents 1 and 2. The optical information-recording medium 200 illustrated here has a configuration consisting of a substrate 201 made of a polycarbonate-based resin and provided with guiding grooves or pits for recording and reproduction, an electronic information-recording layer 202 in which electronic information is recorded with a laser beam 207 applied from the side of substrate 201, a reflection layer 203 made of a metal such as Ag or Au, a protective layer 204 made of a UV-curable resin, a visible information-recording layer 205 in which visible information is recorded with a laser beam 208 applied from the side of the label surface, and an over-coating layer 206 that constitutes the outermost layer, all these layers being provided on this substrate 201 and stacked sequentially in the described order.
As shown in FIG. 2, visible information is recorded in visible information-recording layer 205, for example, by the coloration of a chromogenic organic dye constituting visible information-recording layer 205 upon absorption of a laser beam 208 irradiated from the side of the label surface.
Patent Document 1
JP-A-2000-173096
Patent Document 2
JP-A-2001-283464
Meanwhile, the method of recording visible information on the label surface of a medium with use of a laser beam utilizes changes such as reaction, fusing or deposition caused by heat. Accordingly, sufficient care must be taken for the laser irradiation method because excessive irradiation has a thermal influence on the electronic information having been recorded in the medium. To suppress such thermal influence on the medium, it is necessary to use a low power laser beam. When a low power laser beam is used, laser beam 208 is focused on visible information-recording layer 205 to effectively condense the energy of laser beam 208. Thus, recording with use of low power laser beam 208 becomes possible. To achieve such condensing, in the case where laser beam 208 emitted from the writer for electronic information recording is used, focus servo of laser beam 208 applied from the label surface side is carried out with use of reflected light 209, which is reflected from reflection layer 203 provided on electronic information-recording layer 202.
On the other hand, on visible information-recording layer 205, over-coating 206 constituting the outermost layer is provided. In order to efficiently apply laser beam 208 from the label surface side to visible information-recording layer 205 and to execute a stable focusing, a relatively thick over-coating 206 is preferred; specifically, the thickness of the over-coating preferably lies roughly in the range of 10 to 80 μm. Such over-coating 206 is fabricated, for example, with a UV-curable resin and the like via spin coating or screen printing.
When over coating 206 is formed by coating a UV-curable resin so as to cover the entire area of the label surface of optical information-recording medium 200 in such a manner, it is generally well known that the disk tends to bend by the stretching or compressive stress caused by the curing shrinkage of the UV-curable resin. For example, even a UV-curable resin with a low shrinking ratio and in general use shows shrinkage of 5 to 10%. When over-coating 206 with about 50 μm thickness is formed by using such a UV-curable resin, there occurs a risk of marked deflection of the disk. A method of compensating the resulting deflection by bending the disk in the reverse direction beforehand may be adopted, but such a method requires a precise control for the process of disk manufacture, accompanying disadvantages such as productivity deterioration.